The Montreal Convention covers lost and delayed baggage rules. This convention was incorporated into EU law (Rule 889/2002) and you are entitled to compensation for lost and delayed baggage. The maximum you can claim is approximately £960 which is equivalent to 1,000 Special Drawing Rights, which is a special international currency for airline claims. You can convert SDRs to your home currency using this calculator:
http://coinmill.com/SDR_calculator.html
The problem is this. You do not state how much you claimed. It cannot under any circumstances be more than 1,000 SDR. However, there is no specified amount for delayed baggage, it is subject to negotiation between the customer and the airline. If you are unhappy with their offer and think your claim is reasonable, you can take them to small claims. This is easy to do in the UK, you can do it online via the Court Service website. You can contact them here:
https://www.moneyclaim.gov.uk/web/mcol/welcome
Alitalia must respond to this claim, or you will receive a default judgment. The costs are minimal in this procedure and you will have the pleasure of impounding one of their planes if they refuse to pay.
When it comes down to it, you must be reasonable. If you purchased only necessary items, you can expect the court to be reasonable. If your girlfriend only wears Armani and you purchased £200 worth of underwear, you are unlikely to find the court sympathetic.